(1) Field of the invention
The present invention relates to a system and method for controlling an ignition timing for a vehicular internal combustion engine.
(2) Description of the Background Art
A Japanese Patent Application First Publication No. sho 62-298661 published on Dec. 25, 1987 exemplifies a previously proposed system for controlling an ignition timing for a vehicular internal combustion engine.
In the above-identified Japanese Patent Application Publication, a presence or absence of occurrence of engine knocking is detected whenever the engine is ignited. If the engine knocking occurs, a feedback correction amount is increased by a predetermined amount, e.g., 1 crank angle (CA) for each ignition. If no engine knocking occurs, the feedback correction amount is decreased by the predetermined amount, e.g., 1 CA. In this way, the feedback correction amount is calculated. Then, the above-described feedback correction amount is added to a basic ignition timing read from a basic ignition timing table to derive a final ignition timing.
A U.S. Pat. No. 4,660,535 issued on Apr. 28, 1987, Japanese Patent Application First Publication No. Sho 58-82074 published on May 17, 1983 and Japanese Patent Application First Publication No. Sho 62-96779 published on May 6, 1987 exemplify other previously proposed ignition timing controlling system and method in which so-called MBT (Minimum Advance for a Best Torque) control is carried out to control the ignition timing to be brought at a minimum advance angle for a best torque position, i.e., an MBT point at which a maximum output torque can be derived on the basis of a detection of a peak value position of a combustion pressure.
The MBT control is based on such an engine characteristic that an engine generation torque becomes maximum if the ignition timing is set so that the peak position of the crank angle position at which the combustion pressure becomes maximum reaches a proximity to ATDC 15 (15 After Top Dead Center).
When, e.g., an internal cylinder pressure responsive sensor is used to detect actually the combustion pressure in any one of the engine cylinders, the ignition timing is controlled in the feedback mode so that the peak position of the maximum combustion pressure becomes in the vicinity of ATDC 15. In this case, the required feedback correction amount is calculated on the basis of a deviation between the detected peak position and predetermined crank angle position (set to the vicinity of 15 ATDC as described above) and is added to the basic ignition timing value.
Although in the above-identified U.S. Patent and other Japanese Patent Application Publications the feedback correction quantity is provided for both advance angle side and retardation angle side, combustion noises in the engine becomes largely increased, e.g., when the ignition timing becomes advanced limitlessly with no complete occurrence of knocking. Therefore, a proper limit value of the advanced ignition timing is set at the advance angle side in the actual engine so that no more advance of the ignition timing occurs. (This limit value setting is applied to the MBT control.)
However, since the limit value is fixedly determined according to each of engine operating conditions, the limit value is often set toward a more retardation angle side than an optimum ignition timing for the individual engines in order to place importance on the combustion noise reduction.